This invention relates generally to a process gas flow control system for use in fabricating microelectronic solid-state devices and in other applications, and more particularly to a module which integrates into a single block the various components of the system and their interconnections.
Many circuits and complex multi-stage electronic systems that previously were regarded as economically unfeasible and impractical are now realizable with integrated circuits. The fabrication of a monolithic circuit involves the formation of diodes, transistors, resistors and capacitors on a single substrate, with sufficient isolation between circuit components to minimize parasitic interaction therebetween.
The first phase in fabricating an integrated circuit is a photo resist and oxide-masking process. Because the diffusion constant of typical impurities is much smaller in silicon dioxide than in silicon, a thin layer of silicon dioxide can be used on a silicon wafer to selectively control areas in which diffusion of impurities occurs. In those areas where diffusion into the silicon wafer is desired, the layer of silicon dioxide is removed by etching. If impurity atoms are now applied uniformly to the entire surface and the wafer is heated, diffusion takes place into the exposed silicon, but there is negligible diffusion into the silicon dioxide and the underlying silicon. Thus the silicon dioxide layer acts as a "mask" during the diffusion cycle.
To implement the etching of apertures in silicon dioxide, a photographic technique is employed. The silicon dioxide surface is uniformly coated with a material known as "photo resist." A glass mask containing a pattern of opaque and transparent regions is placed in contact with the photo resist layer, and the assembly then exposed to ultraviolet light. This brings about polymerization in the photo resist layer which makes those areas which have been exposed insoluble in the developing solution. The developer is used to remove the unexposed photo resist.
Exposed photo resist is also insoluble in hydrofluoric acid, which may now be used to remove the areas of silicon dioxide not covered by photo resist. While the hydrofluoric acid removes silicon dioxide, it does not react with silicon. Thus, at this point in the process, certain areas of the silicon surface are exposed, while the remainder are covered with silicon dioxide and photo resist. The photo resist is next removed with hot sulfuric acid and the wafer is now ready for the diffusion process.
Diffusion is performed in an appropriate atmosphere, and a new layer of silicon dioxide is grown on the wafer. Hence at the termination of each diffusion cycle, the entire wafer is covered by silicon dioxide. The areas in which succeeding diffusions occur are controlled by repeating the photo-resist process.
Diffusion is carried out by placing the wafer in a quartz diffusion tube mounted within a furnace operating at a temperature in the order of 1000.degree. C. and higher. In constant-source diffusion, the wafer in the diffusion tube is exposed to a vapor containing a compound with the desired impurity atoms. This compound reacts with the silicon on the surface, yielding impurities in atomic form. As impurity atoms diffuse into the silicon, more are formed at the surface. In limited-source diffusion, a fixed number of impurity atoms/cm.sup.2 is deposited on the silicon surface, and as impurities diffuse into the silicon, they are not replaced at the surface.
Thus in the processing of integrated circuits, oxidizing, reducing and reactive gases are required. It is vital that the gas fed into each diffusion tube in the furnace be of acceptable purity and that the gas be precisely metered. To this end, it has heretofore been the practice to employ a gas flow control system for each gas, the system being adapted to receive the gas from a source at an uncontrolled pressure, to pressure-regulate the gas and to filter it to remove contaminants therefrom. Also necessarily included in the system are means to control the flow rate of the pressure-regulated gas and to indicate the prevailing pressure and flow rate.
The typical gas flow control system for this purpose includes an on-off toggle valve, a gas filter, a diaphragm-type pressure regulator, a pressure gauge, a flow-control valve and a flowmeter or rotameter. In addition to these basic components, the system may optionally include a solenoid valve, a flow switch, a flow controller and a pressure-responsive switch.
The several components which make up the conventional flow control system are mounted on a suitable chassis, fittings and pipes or tubing being provided to intercouple the components to create the necessary flow path. It is essential that all components and all plumbing associated therewith be of materials non-reactive with the gas being metered. In the typical furnace installation, a number of such systems are required, one for each gas being metered. Because each system is constituted by a combination of components and a network of interconnecting pipes, the resultant multi-system arrangement becomes a confusing and intricate maze which presents a jungle-like appearance creating serious maintenance problems.
In order to better organize the combination of components which together constitute a process gas flow control system, attempts have heretofore been made to panel-mount these components in a manner simplifying the required plumbing and minimizing the confusion produced thereby. Among the ordered systems of this type which are commercially-available are those manufactured by Corso-Gray Instrument Inc. of San Jose, California (Models D-3-O and 310) and by Fischer & Porter of Warminster, Pa. (Catalog 80 A 449 for Modular Gas Control Systems).
Though commercially-available gas flow control systems are described as modular, the fact is that they still require relatively elaborate plumbing arrangements which entail pipes and fittings that complicate manufacturing and maintenance procedures.